The present invention relates to an optical waveguide and a method of manufacturing the optical waveguide, and particularly to an optical waveguide including a lower clad layer, core portions formed on the lower clad layer, and an upper clad layer formed to cover the lower clad layer and the core portions, and a method of manufacturing the optical waveguide.
In the field of semiconductor devices, it has been attempted to make the operational speed higher and make the scale of integration larger. For example, significant development has been made to make the performance of microprocessors higher and make the capacity of memory chips larger. To expedite the above development even further, it is required to make the operational speed higher along signal interconnections, make the arrangement density of signal interconnections larger, and improve a delay in signal along electric wiring, and also it is essential to take a suitable measure against EMI (Electro Magnetic Interference) caused by making the operational speed higher and making the arrangement density of signal interconnections larger. As a means for solving the above-described problems, an optical interconnection has been examined. The optical interconnection may be considered to be used in various conditions, typically, between apparatuses, between boards in an apparatus, or between chips in a board. In particular, it may be considered that an optical transmission/communication means using an optical waveguide as a transmission path be suitable for relatively short distance transmission of signals, for example, between chips. In order to get the optical transmission/communication means using the optical waveguide into widespread use, it is important to establish a process of manufacturing the optical waveguide.
The optical waveguide is required to have a small propagation loss of light, and to be manufactured through simple manufacturing steps. With respect to the propagation loss of light, it may be considered that the optical waveguide be manufactured using a material small in propagation loss of light such as quartz. As verified in the case being applied to an optical fiber, quartz is very good in light transmittance, and in actual fact, the optical waveguide manufactured using quartz exhibits a low propagation loss in a range of 0.1 dB/cm or less. The optical waveguide manufactured using quartz, however, problems in requiring a large number of manufacturing steps, in particular, a heat-treatment step at a high temperature of 800.degree. C. or more, and being difficult to ensure a large area of the optical waveguide. For this reason, the optical waveguide has been manufactured using a high polymer material capable of being treated at a low temperature, for example, polymethyl methacrylate or polyimide. Hereinafter, a related art optical waveguide in which core portions made from a high polymer are formed on a substrate will be described with reference to FIGS. 3A to 3D which are schematic diagrams illustrating steps of manufacturing the optical waveguide.
First, as shown in FIG. 3A, a lower clad layer 2 is formed on a substrate 1 made from silicon or glass by spin-coating and necessary heat-treatment.
A core layer 3 having a refractive index larger than that of the lower clad layer 2 is, as shown in FIG. 3B, formed on the lower clad layer 2.
The core layer 3 is, as shown in FIG. 3C, subjected to patterning by photolithography and etching such as RIE (Reactive Ion Etching), to form rectangular core portions 3a to be taken as an optical waveguide pattern.
Finally, as shown in FIG. 3D, an upper clad layer 4 is formed to cover the core portions 3a and the lower clad layer 2 by spin-coating and necessary heat-treatment, to obtain an embedded channel-type optical waveguide.
If the core portions 3a are formed by dry etching such as RIE at the step shown in FIG. 3C, the dry etching is generally performed in an oxygen atmosphere. In such a dry etching, if the RF power becomes large, the surface roughness of each core portion 3a becomes large, and if the gas pressure becomes smaller, the surface roughness of the core portion 3a becomes smaller but the side wall of the core portion 3a tends to be etched. Accordingly, to form the core portions 3a having a small surface roughness and also having highly accurate rectangular shapes, it is required to perform the dry etching at a small RF power and an optimum gas pressure.
However, if the RF power is set at a small value in the above dry etching for forming the core portions 3a to be taken as an optical waveguide, there arise problems that the number of manufacturing steps becomes large for a multi-mode optical waveguide in which the thickness of the core portions 3a is in a range of several tens .mu.m to several hundreds .mu.m, although it does not become large for a single mode optical waveguide in which the thickness of the core portions 3a is in a range of several .mu.m to 10 .mu.m, and that the surface roughnesses of both a side wall of each core portion 3a and the lower clad layer 2 become larger to thereby increase the propagation loss of light passing through the core portions 3a constituting the optical waveguide.